feat(text-splitters): add R programming language support

[gorkachea]

Description:

This PR adds support for the R programming language to LangChain's text splitters, enabling users to intelligently split R code while preserving semantic structure.

Changes:

• Added Language.R enum value to the Language enum
• Implemented R-specific separators in RecursiveCharacterTextSplitter.get_separators_for_language()
• Splits R code along:
  • Function definitions (<- function, = function)
  • Package loading (library(), require())
  • Control flow statements (if(), for(), while(), switch())
  • Data structure creation (data.frame(), list())
  • Standard separators (double newlines, single newlines, spaces)

Why This Matters:

R is widely used in data science, statistical analysis, machine learning, and bioinformatics. This change enables users working with R codebases to properly chunk R scripts for RAG applications and build AI assistants that understand R code structure.

Example Usage:

from langchain_text_splitters import Language, RecursiveCharacterTextSplitter

r_code = """
library(dplyr)

calculate_mean <- function(x) {
  mean(x, na.rm = TRUE)
}
"""

splitter = RecursiveCharacterTextSplitter.from_language(
    language=Language.R,
    chunk_size=100,
    chunk_overlap=0
)

chunks = splitter.split_text(r_code)
# Splits intelligently at library calls and function definitions

Testing:
Tested locally with various R code samples including package imports, function definitions (both <- and = syntax), control flow statements, and data structure creation. All separators work correctly with proper regex escaping.

Issue: Fixes #33824

Dependencies: None